Common Duct Questions

a field of flowers

Common Duct Questions

What is Duct Loss?

Duct loss has two different definitions. The first one is a common term used when performing load calculations on homes. In this case duct loss refers to the thermal loss of Btu.s due to where the duct system is located and and how well insulated the duct system is. Duct systems located in attic areas can experience a significant duct loss of 15 to 30 percent depending on how well insulated the ducts are and how well vented the attic is. This type of duct loss, will require more expensive, larger capacity systems that will cost more to operate for the life of the system. This type of duct loss is unavoidable with duct systems located outside the conditioned space of the house … but steps should be taken to minimize the duct loss. The problem can be avoided all together by simply locating all ducting inside the conditioned space of the home.

The other definition of duct loss is also a common term used by Home Energy Raters in reference to conditioned air that leaks into and out of ducts systems. Air leaking into or out of duct systems created pressure differentials between the air inside and outside a home. These pressure differentials cause unconditioned and unfiltered air to infiltrate the home. Conditioned air that leaks from a duct system into an unconditioned space must be replaced by unconditioned air entering the home.

What are the biggest duct system problems?

Good ducting must begin with a good room-by-room heat load calculation that will determine the right amount of heating, cooling and air that is right for each room. Next comes return and supply air placement and grill selection. Then an acceptable duct static pressure must be selected using the equipment manufacturers static pressure air flow information.

With this information, a proper duct system can be designed. Then the duct system must be installed as designed. The biggest problem with duct systems is that most duct systems are designed around the price of the system and not the requirements of the home or the comfort of the home owners.

If a duct installation passes inspection, is it a good duct system?

Not really. Most building codes in the San Marcos Buda, Kyle and New Braunfels area don’t address the air flow side of a duct system. A duct system with terrible air flow can pass code inspection … as long as it is properly sealed and insulated. Home owners assume that if duct system passes inspection, it must be a good duct system. It is important to realize that code inspections to set the benchmark for the minimum level of what is acceptable. When we build to code, we are building to the lowest level allowed rather than the highest possible quality. Heating and air conditioning systems with poor duct systems will not operate at their designed capacity or designed energy rating. Passing a building code inspection is not a bad thing, but just not being a bad thing isn’t necessarily a good thing.

Are Most Residential Central Air Conditioning And Heating Duct Systems Installed Properly?

According to the Consortium for Energy Efficiency, the answer is no. Most residential systems are not installed properly. Studies conducted by the Proctor group have reached pretty much the same conclusions at the same proportions.

  • Equipment is over sized about 47% of the time.
  • Refrigerant charge is too much or too little about 44% of the time.
  • Air flow is inadequate about 70% of the time.

These problems not only reduce comfort levels inside the home, but make heating and air conditioning systems more expensive to operate. Correcting these problems could reduce air conditioning peak demand by as much as 14 % in existing homes and 25 % in new construction.

Air flow problems due to poorly designed and installed duct systems is a big (but little understood) problem in the HVAC industry. A big part of the problem has always been identifying and quantifying duct system problems. Prior to advent of new tools like Total Performance Diagnostics, finding duct system problems was an arduous and time consuming task.

Is flexible duct a good choice for a duct system?

It doesn’t matter what materials a duct system is made of if the design is bad. Duct board and flexible duct are acceptable choices and work well when designed correctly for the materials. The choice does not need to be a trade-off between the higher material and labor costs of metal ducts and the performance characteristics of other systems.

Flexible ducting is inexpensive and easy to install, but because of its spiral wire helix construction, flex duct has a higher friction loss when compared to sheet metal duct or fiberglass duct board. The inner core of flex duct changes shape with compression and bending, which increases turbulence and friction loss. Unfortunately compression and bending are the very attributes that make flex duct so popular and easy to use.

When installed and designed correctly flexible spiral duct can and does work well. The biggest problem is not the material, but how it is installed. It is true that when it comes to air conditioning and heating ducts … “no duct job is so poorly done … that someone can’t do it cheaper”.

Can a bad duct system lower the quality of the air inside a home?

Yes! There are several way poor duct systems may work to lower indoor air quality.

  1. Unsealed return air ducts may allow unfiltered air to enter the home bringing with it heat, humidity, dirt, spores, allergens and other materials.
  2. Unsealed supply air ducts may release conditioned air into unconditioned spaces, creating a net negative pressure inside a home causing unfiltered to enter the home by way of any crack, crevice, or unsealed opening in the structure.
  3. Ducts with excessively high static may reduce system air flow, creating dew point conditions within the duct system resulting in excess moisture that created a breeding ground for microbial growth in both the ducts and air handling equipment.
  4. Duct systems with poor, inadequate or compromised insulation may also exacerbate the growth of microbial organisms.

Duct tape is good for fixing everything, including sealing ducts, right?


Late in 1998, scientists at the Lawrence Berkeley Nationa l Laboratory made a startling discovery — ever-popular duct tape was useful for hundreds and hundreds of tasks, but holding ducts together wasn’t one of them.

a roll of duct tape

Over three months, researchers tested duct tape and 31 other sealants under accelerated laboratory conditions that mimicked long-term use in the home. They heated air to nearly 170 degrees and chilled it to below 55 degrees before blasting it through ducts. They baked ductwork at temperatures up to 187 degrees to simulate the oven-like conditions of a closed attic under a hot summer sun.

Of all the things they tested, only duct tape failed — and they reported it failed reliably and often quite catastrophically.

Instead of using duct tape, the researchers recommended sealing ducts with mastics, gooey sealants that are painted on and allowed to harden. Metal ducts should be held together with sheet metal screws; flexible duct connections should be secured with metal or plastic bands.

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Does “Air Flow” really make a difference in HVAC system performance?

Absolutely! Equipment SEER is based on the manufacturer’s efficiency rating with duct resistance factored in for the duct system. A bad duct system causes the equipment to work harder and less efficiently. Poor ducting may also effect indoor air quality and total home comfort.